1. Technical Field
The present invention relates to a method for manufacturing an image sensor, and more particularly, to an image sensor having a gate spacer, which can prevent damage to a photodiode region in the image sensor.
2. Discussion of the Related Art
An image sensor is a semiconductor device that can convert an optical image into an electrical signal. A Charge Coupled Device (CCD) is a device in which individual Metal-Oxide-Silicon (MOS) capacitors are located very close to each other to store and carry charge carriers. A CMOS image sensor is a device that has the same number of MOS transistors as the number of pixels and employs a switching method in which outputs are detected sequentially using the MOS transistors. Here, the MOS transistors are formed with CMOS technology in which a control circuit and a signal processing circuit are used as peripheral circuits.
Such an image sensor has a Color Filter Array (CFA) arranged on an optical detection portion that receives external light and generates and stores photo-generated charges. The color filter array includes red, green, and blue color filters or yellow, magenta, and cyan color filters. The image sensor is composed of an optical detection portion which detects light and a logic circuit portion which processes the detected light into an electrical signal and creates corresponding data. Although efforts have been made to increase a fill factor, which is the ratio of the area of the optical detection portion to the entire area of the image sensor, in order to increase optical sensitivity, there are limitations in the efforts with the restricted area since it is not possible to essentially remove the logic circuit portion.
A focusing technology has been introduced to change the path of light incident on areas other than the optical detection portion and thus to focus the incident light on the optical detection portion. A method of forming microlenses on the color filters in the image sensor is used to accomplish this focusing.
FIG. 1A is an equivalent circuit diagram of a unit pixel (dashed in the drawing) of a conventional CMOS image sensor.
As shown in FIG. 1A, the unit pixel includes one photodiode PD and four NMOSs Tx, Rx, Sx, and Dx (specifically, a transfer transistor TX, a reset transistor Rx, a select transistor Sx, and a drive transistor Dx). The transfer transistor Tx transfers photo-generated charges collected at the photodiode PD to a floating diffusion (FD) region. The reset transistor Rx sets voltage at the node to a desired level and discharges charges Cpd to reset the floating diffusion region. The drive transistor Dx serves as a source follower buffer amplifier. The select transistor Sx is switched to allow addressing.
Each of the transfer and reset transistors Tx and Rx use a native NMOS transistor. Each of the drive and select transistors Dx and Sx uses a normal NMOS transistor. Reset transistor Rx is a transistor for Correlated Double Sampling (CDS).
In the unit pixel of the CMOS image sensor as shown in FIG. 1A, light in the visible wavelength range is detected at the photodiode region (PD) using the native transistor and the detected photo-generated charge is transferred to the floating diffusion region (FD) and an electrical signal corresponding to the amount of the photo-generated charge provided to the gate of the drive transistor Dx is output through an output terminal of the unit pixel.
FIG. 18 is a sectional view of a conventional CMOS image sensor device. This figure shows only a portion of the device corresponding to both the photodiode PD and the transfer transistor Tx which transfers photo-generated charges collected at the photodiode PD to the floating diffusion region PD. The following is a brief description of a method for manufacturing an image sensor with reference to FIG. 1B. A p-type epitaxial layer 12 doped with a low-concentration p-type impurity is grown on a p+ substrate 11 doped with a high-concentration p-type impurity and a field oxide layer 13. Separation between unit pixels is formed on a specific portion of the p-type epitaxial layer 12 using a local oxidation of silicon (LOCOS) method.
A gate electrode 14 of a transfer transistor Tx is formed on the p-type epitaxial layer 12. Although not shown, gate electrodes of a drive transistor Dx, a reset transistor Rx, and a select transistor Sx are formed simultaneously at this time.
An n-type diffusion layer 15 is formed in a portion of the p-type epitaxial layer 12 at one side of the gate electrode 14 of transfer transistor Tx by implanting low-concentration n-type impurity ions with high energy into the epitaxial layer 12. An insulation layer for spacers is then deposited over the entire area of the substrate and blank etching is performed on the insulation layer to form a spacer 16 in contact with either sidewall of the gate electrode 14 o£ the transfer transistor Tx.
Subsequently, low-energy p-type impurity ions are implanted, for example, using a blanket ion implantation method, to form a p-type diffusion layer 17 in a top portion of the n-type diffusion layer 15 and a surface portion of the p-type epitaxial layer 12. Here, the p-type diffusion layer 17 formed in the n-type diffusion layer 15 is separated from gate electrode 14 as far as the thickness of the spacer 16.
A shallow pn junction including the p-type diffusion layer 17 and the n-type diffusion layer 15 is formed through such implantation of low-energy p-type impurity ions. As a result, a pnp type photodiode including the p-type epitaxial layer 12, the n-type diffusion layer 15, and the p-type diffusion layer 17 is formed.
In the conventional image sensor including the pnp type photodiode region, blank etching is performed on the insulation layer to form the spacer 16 in contact with either sidewall of the gate electrode 14 of the transfer transistor Tx as described above. In this case, the photodiode region A may be excessively corroded as shown in FIG. 1C to open the photodiode, thereby causing damage to the image sensor and thus degrading the performance of the image sensor.